Typically, a field effect transistor (“FET”) gas sensor measures a change in a work function between a gas-sensitive material and a reference material. FIG. 6 is a schematic illustration of such a FET gas sensor 10. A gateless transistor 12 is capacitively coupled, through an air gap 14, to a gas-sensitive layer 16. The air gap 14 is configured as a gas inlet a few micrometers (μm) wide. The gas-sensitive layer 16 is adapted such that its surface potential changes upon contact with a gas within the air gap 14, and thus controls the transistor 12. The working temperatures of such a sensor are between ambient temperature and 150° C.
FIG. 7 is a diagrammatic illustration of a typical suspended gate FET (“SGFET”) gas sensor. One example of such a SGFET gas sensor is disclosed in German Patent No. DE 4239319 C2, which is hereby incorporated by reference in its entirety.
FIG. 8 is a diagrammatic illustration of a typical CCFET gas sensor 17. One example of such a CCFET gas sensor is disclosed in German Patent No. DE 4333875 C2, which is hereby incorporated by reference in its entirety. Referring again to FIG. 8, an air gap 18 and a gas-sensitive layer 20 are separated from a readout FET structure 22. The readout FET 22, which includes a source contact 24 and a drain contact 26, is controlled by an uncontacted gate 28. The uncontacted gate 98 and a gas-sensitive electrode 10 form a capacitor system.
Disadvantageously, the aforesaid gas sensors frequently measure reactions from interfering gases (i.e., non-target gases) within the air gap. “Undesired” signals from the interfering gases may be superimposed on a “desired” signal from a reaction of a target gas. As a result, this superposition may distort measurements of the target gas. In addition, these interfering gases may decrease the signal level of the target gas.
To increase accuracy of these FET gas sensors, several methods and systems have been developed to reduce the effects of interfering gases. For example, improvements may include (1) optimizing sensitive materials, (2) compensating for the interfering gases using a reference sensor, and (3) using a filter to suppress effects of the interfering gases.
First, sensitive materials in a FET gas sensor may be optimized. This may be accomplished by optimizing gas-sensitive materials and reference materials located on the transistor of the sensor to selectively reduce the influence of interfering gases.
Second, interfering gases may be compensated for by using a reference sensor. For example, a second sensor (i.e., a reference coating) sensitive to a known interfering gas may be used to compensate for the influential effects of the interfering gas on the measurement of the target gas. However, similar to the first sensor (i.e., the gas-sensitive material for the target gas), this second sensor may have a limited sensitivity. That is, the second sensor may also be affected by cross sensitivities. In addition, the second sensor may have a minimal effect where the level of the interfering signal exceeds the useful signal by a multiple.
Third, a filter may be used to suppress interference from interfering gases even where the interfering gases are permeable to a target gas. For example, an activated charcoal filter may remove an interfering gas. However, use of such a filter may prove problematic during long term operations. For example, the interfering gas may break through the filter when the limited capacity of the filter has been reached. In another example, where the sensor is operated at an elevated temperature, a catalytic filter may be used to transform an interfering gas, via a chemical reaction, into one or more non-interfering components (i.e., components that do not react with the gas-sensitive material to produce a signal). An example of a catalytic filter for decomposing alcohols is disclosed in German Patent No. DE 4310914, which is hereby incorporated by reference in its entirety. Although these catalytic filters may not exhibit the aforesaid filter capacity problems, they typically require temperatures above 300° C. Therefore, sensors using catalytic filters may not include structural elements with silicon (Si) chips that typically have a maximum operating temperature of about approximately 150° C.